In the past, airfoils such as blades and vanes, for gas turbine engines have been manufactured in longitudinally split halves with the halves diffusion bonded together to form a hollow airfoil. In this technique, each half of the airfoil typically includes one or more support ribs which in the bonded airfoil are bonded internally to another opposed rib from the other half of the airfoil or to the inner side of the other airfoil half. Spaces between the ribs provide internal passages for cooling fluid such as compressor discharge cooling air flow.
In the past, the support ribs have extended from the airfoil forming skin of the airfoil halves 3, 3' substantially normal or perpendicular thereto; e.g., as shown in FIGS. 1 and 2 where internal support ribs 1, 1' are shown extending substantially normal to the airfoil-forming skins 2, 2' of the respective airfoil halves 3, 3' along the length of the ribs from adjacent the root 4 toward the tip 5 of the airfoil. It is apparent from FIGS. 1 and 2 that the airfoil of each half is highly twisted transversely along its longitudinal axis from adjacent the root 4 toward the tip 5.
As a result of the high transverse twist from root to tip of the airfoil and the orientation of the support ribs 1, 1' substantially normal to the skins 2, 2', the support ribs 1, 1', including their bonding surfaces 1a, 1a', twist or spiral transversely from the root toward the tip and complicate the bonding process to join airfoil halves 3 and 3' at the bonding surfaces provided on the ribs. In particular, due to the high twist of the support ribs, the bonding dies receiving the mated airfoil halves will require a very close fit between mating die parts to maintain accurate bonding alignment between the rib bonding surfaces and very low bonding loads or forces must be used to press the dies and airfoil halves together to prevent deformation of the rib bonding surfaces 1a 1a' which are not normal to the direction of the bonding pressure. As a result of the low bonding pressures capable of being applied, longer bonding times are required to diffusion bond the airfoil halves together; e.g., typical bonding times could be 4-6 hours with airfoil components having transversely twisted ribs.
Another disadvantage of the prior art technique described with the support ribs substantially normal to the airfoil skin and twisted transversely along their lengths is that the airfoil halves are not amenable to being forged to near net shape as a result of the high transverse twist of the support ribs in the longitudinal direction along the airfoil, although such airfoil halves have been cast or machined in the past by well known methods.
What is needed is an airfoil structure amenable to fabrication in halves or split parts by forging and bondable together in relative short times.
Prior art patents illustrating a twisted airfoil having internal support ribs extending substantially normal to the airfoil skin and thus having highly transversely twisted support ribs are U.S. Pat. No. 4,512,069 issued Apr. 23, 1985, to K. Hagomeister and U.S. Pat. No. 4,501,053 issued Feb. 26, 1985 to H. Craig.